I. Technical Field
The present invention relates to a hydrodynamic bearing type rotary device using a hydrodynamic bearing and a recording and reproducing apparatus including the same.
II. Background Art
In recent years, recording and reproducing apparatuses and the like using discs to be rotated have experienced an increase in memory capacity and an increase in the transfer rate for data. Thus, bearings used for such recording and reproducing apparatuses are required to have high performance and high reliability to constantly rotate a disc load with high accuracy. Accordingly, hydrodynamic bearings suitable for high-speed rotation are used for such rotary devices.
The hydrodynamic bearing type rotary device has a lubricant, such as oil between a shaft and a sleeve, and generates a pumping pressure by hydrodynamic grooves during rotation. Thus, the shaft rotates in a non-contact state with respect to the sleeve in the hydrodynamic bearing type rotary device so it is suitable for high-speed rotation.
Hereinafter, an example of conventional hydrodynamic bearing type rotary devices will be described with reference to FIGS. 13 through 15. As shown in FIG. 13, a conventional hydrodynamic bearing type rotary device includes a sleeve 21, a shaft 22, a stopper 23, a bottom plate 24, oil 25, a hub 27, a base plate 28, a rotor magnet 29, a stator 30, and a disc 31.
The shaft 22 is press-fitted to the hub 27. The shaft 22 is inserted into a bearing hole 21A of the sleeve 21 so as to be rotatable. On at least one of an outer peripheral surface of the shaft 22 and an inner peripheral surface of the sleeve 21, radial hydrodynamic grooves 21B are formed to form a radial bearing surface. On a surface of the sleeve 21 opposing a lower surface of the hub 27, thrust hydrodynamic grooves 21D having a spiral pattern as shown in FIG. 14 are formed to form a thrust bearing surface. The bottom plate 24 shown in FIG. 13 is adhered to the sleeve 21. The sleeve 21 has a flange portion 21C on an outer peripheral surface on the side facing the hub 27. The flange portion 21C has a large diameter and a tapered surface 21E on a surface on the side of the base plate 28. An oil reservoir 26 is provided between the tapered surface 21E and a substantially circular stopper 23 fixed to the hub 27. The stopper 23 is engaged to the flange portion 21C of the sleeve 21. The oil 25 is sealed in the bearing cavity entirely, and a gas-liquid interface is formed near the oil reservoir 26.
To the base plate 28, the sleeve 21 is fixed. The stator 30 is also fixed to the base plate 28 so as to oppose the rotor magnet 29. Magnetic centers of the rotor magnet 29 and the stator 30 in an axial direction are largely shifted in the axial direction. Thus, the rotor magnet can generate an attraction force in a direction indicated by arrow A in the figure. To the hub 27, the rotor magnet 29 and the disc 31 are fixed.
Operations of the conventional hydrodynamic bearing type rotary device having the above-described structure are as follow. In the conventional hydrodynamic bearing type rotary device shown in FIG. 13, when an electric current is supplied to a coil wound around the stator 30, a rotary magnetic field is generated, and a rotary force is applied to the rotor magnet 29. Thus, the rotor magnet 29 starts to rotate with the hub 27, the shaft 22, the stopper 23, and the disc 31. When these members rotate, the hydrodynamic grooves 21B gather the oil 25 filled in the radial gap to generate a pumping pressure between the shaft 22 and the sleeve 21, forming a radial bearing. The thrust hydrodynamic grooves 21D gather the oil 25, and generate a pumping pressure in a thrust direction between the hub 27 and the sleeve 21. The rotating part is caused to float in a direction opposing the attraction force of the rotor magnet 29 which is indicated by arrow A in the figure, and is started to rotate in a non-contact state.
As described above, the shaft 22 can rotate in a non-contact state with respect to the sleeve 21 and the bottom plate 24. With a magnetic head or an optical head (not shown), data can be recorded/reproduced to/from a rotating disc 31.
However, in the above conventional hydrodynamic bearing type rotary device, air 32 may be trapped in a gap between a lower end surface of the hub 27 and the sleeve 21, as shown in FIG. 15 (S1 and S2 in the figure) and may not be discharged.
More specifically, in a hydrodynamic bearing, a pressure in the bearing cavity varies due to functions of hydrodynamic grooves 21D and 21B. When a large amount of air is trapped within the bearing cavity, the air 32 may expand due to pressure change, and cause the oil 25 to flow out from the oil reservoir 26. Once the oil 25 flows out from the oil reservoir 26, it may result in oil film rupture in the hydrodynamic grooves 21D and 21B. In such a case, problems, such as the required performance not being achieved, the bearing being worn and broken, and the like may occur.
It has been recognized that, if the maximum gap S1 is too large, bubbles (air) 32 tend to accumulate. However, actually, the cause of the bubbles being trapped in the bearing cavity cannot be attributed simply to the width of the gap. Conventionally, it has been impossible to anticipate movement which ensures the bubbles 32 to be discharged or to explain how easy the bubbles 32 can be trapped in the bearing.